ABSTRACT The goal of this application is to determine the role of trophic interactions in the susceptibility (or resilience) to the effects of aging on the neuromuscular system. Age-related neuromuscular dysfunction is an important determinant of overall health, limiting independence, increasing frailty and predisposing individuals to age- related co-morbidities and mortality. Interactions between motoneurons and the muscle fibers they innervate determine muscle fiber properties and have a significant impact on muscle function throughout the lifespan. Motoneuron-muscle fiber interactions are likely exerted via trophic factors that vary across muscle groups. Brain-derived neurotrophic factor (BDNF) acting via its high-affinity receptor tropomyosin related kinase receptor (TrkB) has a known role in the maintenance of the adult NMJ. However, the role of BDNF/TrkB signaling in old age is not presently understood. Exciting recent studies show that inhibition of TrkB kinase activity exerts deleterious effects on neuromuscular transmission that vary across age groups, replicating the effects of old age at young NMJs. The current proposal will use a combination of highly-innovative methods to explore mechanistically the role of disrupted trophic factor signaling at the NMJ in old age. Our working hypothesis is that susceptibility to age-related neuromuscular dysfunction depends on motoneuron-muscle fiber trophic influences exerted by BDNF/TrkB signaling at the NMJ (aim 1) and motoneuron (aim 2). Furthermore, trophic factors can determine susceptibility to neuromuscular damage resulting from autophagy imbalance causing accumulation of protein aggregates and degeneration in old age. Two specific aims are proposed: Specific Aim 1) To determine the cellular (trophic factor dependent) mechanisms underlying susceptibility to neuromuscular dysfunction in old age. Specific Aim 2) To determine the molecular (trophic factor dependent) mechanisms underlying age-related effects at motoneurons. These results constitute the necessary foundation for the development of targeted therapies to mitigate aging effects on neuromuscular performance and increase the health span in the aging population with therapies initiated later in life to combat frailty and disability.